Scanning electrochemical microscopy (SECM) was used to study the reaction mechanism of the oxidation of natural antioxidants (ascorbic acid and chlorogenic acid) at oil/water (O/W) interfaces. The apparent first-order rate constants (k(f)) were determined for the electron transfer (ET) between the antioxidant in the W phase and the radical cation of 5,10,15,20-tetraphenylporphirinato zinc(II), which was generated on an ultramicroelectrode immersed in the O phase. For both the antioxidants, the values of k(f) showed a linear dependence on the square root of the antioxidant concentration in the W phase. The kinetic analysis based on the reaction layer theory suggested a new type of ion transfer (IT) mechanism, in which the ET reaction occurs homogeneously in the O phase (not in the W phase as in the previous IT mechanism). (c) 2007 Elsevier B.V. All rights reserved.